Phthalic anhydride (PAN) and maleic anhydride (MAN) are important commercial chemicals useful in the manufacture of plasticizers, polyesters, alkyd resins and dyes.
Maleic anhydride is typically produced by air oxidation of butane, butene or benzene (e.g., 2-5 mole percent in air) in the presence of a vanadium/phosphorus catalyst. Selectivities are typically 60-70 mole percent for conversions ranging from 80 to approximately 100%. This oxidation process produces organic by-products such as light acids (e.g., acetic acid and acrylic acid).
A phthalic anhydride and maleic anhydride mixture is typically produced by oxidation of n-pentane in the presence of a selective oxidation catalyst, e.g., a vanadium phosphate oxide catalyst (VPO) or a molybdenum oxide catalyst. Vanadium phosphate oxide catalysts can be obtained, for example, from precursors prepared by either of two methods: (1) immediate precipitation of a solution containing vanadia in isobutanol and H.sub.3 PO.sub.4 and (2) facilitating, before precipitation, the conditions for the intercalation of the isobutanol in the VOPO.sub.4 hydrated phase. Catalysts were obtained from the precursors by in situ treatment under reaction conditions for the selective oxidation of n-pentane. Control of the stage of formation of the precursor is crucial for obtaining a selective catalyst for formation of phthalic anhydride (PAN). The preparation of VOHPO.sub.4.1/2H.sub.2 O via a full development of the VOPO.sub.4.2H.sub.2 O phase, containing intercalated isobutanol, seems to favor the adequate structure of the precursor which promotes the formation of PAN. By careful control of the preparation of the VPO precursor, e.g., controlling the isobutanol/water ratio, the final catalyst can lead to the desired PAN/MAN ratio.
The resulting vapor phase oxidation product from the catalytic air oxidation of butane, butene or benzene typically comprises: a reaction gas composed of nitrogen, oxygen, water, carbon dioxide, carbon monoxide, acetic acid, acrylic acid, maleic anhydride, maleic acid, and partial oxidation products. While the vapor phase oxidation product from the catalytic air oxidation of pentane typically comprises: a reaction gas composed of nitrogen, oxygen, water, carbon dioxide, carbon monoxide, maleic anhydride, acetic acid, acrylic acid, phthalic anhydride, and other partial oxidation products.
The aforementioned vapor phase oxidation products are typically first cooled to generate steam and then delivered to expensive switch condensers, where they are cooled to permit the desublimation of a crude phthalic anhydride from the gas. Thereafter, the crude phthalic anhydride is sent to a finishing section in order to produce substantially pure phthalic anhydride; whereas the crude maleic anhydride is taken overhead from the switch condenser as a vapor and then recovery by solvent absorption.
The switch condensers operate alternatively on cooling and heating cycles in order to first collect either the maleic anhydride or, in the case of a PAN/MAN mixture, phthalic anhydride as a solid and then melt it for removal from the condensers. The use of switch condensers to separate crude phthalic anhydride from a vapor phase oxidation product is described in U.S. Pat. No. 5,214,157, which is incorporated herein by reference. Typically, the reactor vapor phase oxidation product is cooled close to the solidification point 131.degree. C. (268.degree. F.) of phthalic anhydride and any condensed liquid is separated out before the remaining vapor enters the switch condensers. The switch condensers desublime the vapor phase oxidation product using the cold condenser oil, and then melt off the solid phase crude phthalic anhydride product using a hot condenser oil heated with steam.
A substantial amount of impurities exit switch condensers as part of the vapor stream, whereas the crude maleic anhydride or phthalic anhydride/maleic anhydride product is plated out on the heat exchange tubes as a solid during the cooling step and exits the switch condensers at the bottom as a liquid during the melting step. The vapor gases from the switch condensers are sent to waste gas incinerators where the by-products are destroyed by oxidation to carbon dioxide and water.
Unfortunately, switch condensers contribute to a significant portion of the capital and operating costs of a phthalic anhydride plant. Also, switch condensers operate in a batch mode on 3-6 hour cycles to desublime solid phthalic anhydride on the heat exchange tubes. Another problem associated with switch condensers is that they necessitate frequent maintenance which requires that designated switch condensers be taken out of service on a periodic basis. Maintenance of switch condensers is costly due to the high labor requirement and condenser down time.
The present inventors have developed a unique process scheme which avoids the need to use expensive switch condensers to recover either maleic anhydride or phthalic anhydride/maleic anhydride mixture from vapor phase oxidation products. This unique process continuously condenses and recovers phthalic and maleic anhydride by rectification without the formation of an intermediate solid phase, wherein the maleic anhydride is taken overhead and recovered from the other overhead by-products by means of distillation. The rectification tower can be operated with or without the aid of a solvent which lowers the freezing point of the mixture contained therein so as to avoid freezing of the overhead products in the top of the rectification tower and/or condenser. A liquid phthalic anhydride recovery process using a rectification tower is disclosed in co-pending U.S. patent application, Ser. No. 08/431,647, (Dengler et al.), filed on May 2, 1995 now U.S. Pat. No. 5,731, 443, and which is incorporate herein by reference. The liquid phthalic anhydride recovery process disclosed in U.S. patent application, Ser. No. 08/431,647, (Dengler et al.) is based on concentrating the indigenous maleic anhydride (MAN) and benzoic acid (BA) co-products produced in the phthalic anhydride (PAN) reactor to form a minimum freezing MAN/BA/PAN mixture in the rectification tower condenser system. The present inventors have discovered that this condensing temperature is critical to the liquid phthalic anhydride recovery process and must: (1) be low enough to recover sufficient MAN to insure the liquid distillate will not freeze; and (2) be high enough to avoid free water from condensing thereby forming excessive amounts of acids in the distillate.
However, Dengler et al. does not pertain to the recovery of maleic overhead by means of distillation, rather it discloses a process for recovering maleic as a liquid from a vapor phase oxidation product which comprises mixing the vapor phase oxidation product with reaction by-products in a contacting means such that a substantial portion of the maleic anhydride contained within the vapor phase oxidation product transfers from the vapor phase to a liquid phase and the by-products contained in the reaction by-products which are more volatile than maleic anhydride transfer from the liquid phase to the vapor phase.
Dengler et al. also does not describe or suggest the use of a solvent to lower the freezing point of the mixture contained within the rectification tower. Therefore, the present invention provides a unique method for recovering large quantities of phthalic anhydride without the need for expensive switch condensers.
A similar conventional technique for recovery maleic anhydride is to scrub the reaction off-gas with a solvent to remove maleic anhydride before the off-gas is exhausted to an incinerator. The rich solvent stream is heated and vacuum stripped to release the maleic anhydride. Crude maleic anhydride is condensed and sent to purification. Stripped solvent is cooled and returned to the maleic anhydride absorber. A solvent slipstream is withdrawn for purification. Thereafter, the crude maleic anhydride is fractionated to remove light ends. The small quantity of by-product light ends is delivered to the incinerator for destruction and waste heat recovery. The maleic anhydride is further fractionated to separate any solvent and heavies which accompanied it in the stripper overhead. The bottoms stream is returned to the maleic anhydride stripper. A special step is included to remove any solvent degradation products from the slipstream, in order to prevent the build-up of impurities in the solvent recycle loop.
The process for absorption in an organic solvent as discussed immediately above is very expensive in terms of both hardware and consumable organic solvent. The maleic anhydride recovery using distillation with or without a solvent provides a significant advance in terms of cost and processing time verses the conventional solvent absorption process. The unique process according to the present invention uses distillation as a maleic anhydride recovery technique. The process according to the present invention recovers maleic anhydride from any gas stream resulting from any of the current oxidation processes, without the restrictions of the current technology.
The substantial technical differences between using absorption versus rectification for separating out maleic anhydride from a vapor phase oxidation gas product without the formation of an intermediate solid phase can be understood by comparing the vapor to liquid weight ratios (V/L) in the absorbent tower against the V/L for the rectification tower. For example, the V/L for the absorbent tower U.S. Pat. No. 4,285,871 (Keunecke '871), as calculated from the example provided therein is 0.3 to 0.7. The rectification tower of the present invention exhibits a V/L ratio of between 5 to 20, more preferably 8 to 15. That is, due to the substantial pumparound or recycling of the bottoms stream which is required in any absorbent case, its V/L ratio is only a fraction of that which occurs during rectification. The low V/L ratio in the absorbent case of Keunecke '871 clearly demonstrates that due to these high pumparound rates the absorbent tower is not providing any noticeable degree of separation of liquid maleic anhydride from a vapor phase maleic anhydride, such as that recited in the present invention.
Therefore, the advantages of the recovery processes of the present invention over the conventional solvent absorption processes discussed above are in simplification of process concept and elimination of adsorption and stripping steps required for the commercial solvent absorption process.
The present invention also provides a unique method for recovering maleic anhydride from a vapor phase oxidation product formed from the air oxidation of butane, butene and benzene by taking the maleic anhydride overhead as a vapor stream. This recovery process utilizes a uniquely tailored solvent which enlarges the window of operation for the condenser by reducing the freezing point of the condenser reflux, compared to the freezing point of maleic anhydride, and allows the condenser to operate at a lower temperature; thereby reducing the amount of maleic anhydride in the vapor stream exiting the condenser.
The solvent, taken overhead and condensed with the maleic anhydride in the condenser, forms a condensate which provides the liquid reflux to the rectification tower. In the rectification tower, the liquid reflux stream is stripped of the maleic anhydride by the hot vapor phase oxidation product and the residual solvent in the liquid phase is removed as a bottoms stream from the tower. The vapor phase maleic anhydride, recovered as liquid in the condensate, is then distilled from a portion of this liquid phase and the residual solvent from this distilled portion of the condensate stream is returned to the rectification tower. The liquid condensate from the condenser also contains low boiling, compared to maleic anhydride, by-products from the oxidation reaction such as acetic acid and acrylic acid.
Further, the use of a tailored solvent-enhanced recovery process is beneficial when maleic anhydride and phthalic anhydride are co-produced as a vapor phase oxidation product. The present inventors have discovered that the addition of a small amount of a selected solvent to a MAN/PAN mixture substantially reduces the freezing point of the total mixture. More importantly, the present inventors have discovered that the addition of a solvent to concentrations of 5 to 10 mole percent to the MAN/PAN mixtures obtained as the liquid reflux to the rectification tower is sufficient to reduce the freezing point of the MAN/PAN mixtures and allows recovery of both MAN and PAN by means of simple distillation.
The present inventors have developed a modification to the liquid phthalic anhydride recovery process disclosed in co-pending U.S. patent application, Ser. No. 08/431,647, (Dengler et al.). The present invention provides for the addition of a solvent to the rectification tower reflux system to lower the freezing point of the PAN/MAN mixture. In addition, the solvent according to the present invention will increase the low temperature operating region and broaden the potential application of the maleic anhydride or phthalic anhydride/maleic anhydride recovery process
In summary, using a specifically tailored solvent in the maleic anhydride or PAN/MAN mixture recovery process: (1) allows for the choosing of a tailored component with properties better than maleic anhydride (MAN) reflux to reduce the freezing point of the maleic anhydride/phthalic anhydride mixture; (2) provides a low freezing point operating region sufficiently large for good commercial plant operation and control; and (3) frees the liquid recovery process of the present invention from depending on the maleic anhydride and other reaction by-products to lower the freezing point of the liquid condensate, the rectification tower reflux, sufficiently to recover either maleic anhydride (MAN only) or maleic anhydride and phthalic anhydride with high efficiency by simple distillation means.